Generally speaking, an optical integrated circuit is constructed of a complicated combination of structures, as shown in FIG. 1. On a substrate (of, for example, gadolinium-gallium-garnet: GGG) 1, more specifically, an optical waveguide layer (of, for example, yttrium-iron-garnet: YIG) 2 which has a higher refractive index than that of the substrate 1 is formed, and optical waveguide strips (called "ridges") 3 are formed thereon. Light is transmitted through the ridges so that, when two ridges are provided adjacent to each other, as shown in FIG. 1, all the optical energy of the light propagating through ridge 3-1 is transferred to ridge 3-2 within a longitudinal range that is determined by the shape, refractive index, etc., of the ridges. In this case, transfer to the ridge 3-2 can be prevented by placing magnets 4-1, 4-2 which apply magnetic fields to the ridges on the tops of the ridges, as shown in FIG. 2(a), and by controlling the intensities of the magnetic fields thereof so that the light propagating through the ridge 3-1 can emanate from an output terminal for the ridge 3-1. It is customary to use serpentine coils (i.e., wound coils) instead of the magnets to control the magnetic fields. In this case, it is difficult to form the coils directly onto the ridges, and a flat medium must be formed on the ridges before the coils can be formed on the flat medium. On the other hand, when the substrate 1 is made of an electro-optical crystal (e.g., gallium arsenide, GaAs) and the optical waveguide layer 2 is made of an electrooptical crystal which has a higher refractive index than that of the first crystal (e.g., GaAs with a higher resistance), electrodes 5-1 and 5-2 are attached to the tops of the ridges, as shown in FIG. 2(b), and an electrode 6 is attached to the bottom of the substrate so that the transfer of light can be prevented by a control of the voltage between the two terminals. This is the principle of an optical switch within an optical integrated circuit. The former type of switch employs a magneto-optical effect whereas the latter employs an electro-optical effect. Generally speaking, an optical integrated circuit makes use of the electromagnetic effect. FIG. 1 shows the simplest case thereof, but an optical integrated circuit usually has a complicated shape. For example, the optical integrated circuit can have a construction such as that shown in FIG. 3.
A problem with a complicated optical integrated circuit which has such curved optical waveguides concerns how much the optical transmission losses can be reduced. A special problem is losses due to optical scattering. These scattering losses are partly caused by irregularities due to thermal fluctuations in the material making up the optical waveguides, and partly by the structure of the optical waveguides themselves. The former type of scattering is determined by the material, and results in a loss of about 0.8 dB/km for an optical wavelength of 1 .mu.m, which is so small that it can be neglected in an optical integrated circuit. However, the latter type of scattering varies so much according to the method by which optical waveguides are manufactured that it raises a very serious problem.
In order to form the ridges, a wet-etching method or a dry-etching method is usually used. In the wet-etching method, the desired portions of the YIG or the like are etched with hot phosphoric acid; and in the dry-etching method, the desired portions of the YIG are mechanically etched by argon ions (Ar.sup.+) striking the YIG. The sides of the ridges prepared by these methods are irregular, as indicated at 7 in FIG. 4. These irregularities 7 are caused by unevenness in the etching and the material used, or by irregularities in the photo-mask used to form the ridges. The light propagating through the ridges is scattered optically by the irregularities of their sides, so that an optical transmission loss results. This optical transmission loss .alpha. is more or less proportional to (n.sub.1.sup.2 -n.sub.3.sup.2), where n.sub.1 is the refractive index of the ridges and n.sub.3 is the refractive index of the surrounding medium, and is strongly influenced by the period of the irregularities in the ridge sides. These irregularities in the ridge sides are currently about 0.08 .mu.m. The resultant optical loss at the straight portions of the ridges is equal to or less than 1 dB/cm. The reason for this low transmission loss is that the penetration of optical energy (i.e., the electrical field distribution E) propagating through the ridges to the outside is small at the straight portions, as shown in FIG. 5, so that the irregularities in the ridge sides have only a small effect. As shown in FIG. 5, however, there is a large leakage of optical energy (i.e., the electrical field distribution E) to the outside at the curved portions, so that the irregularities in the ridge sides have a strong effect and increase the optical scattering.
In order to make the influence of the irregularities more apparent, optical integrated circuits were fabricated by making each substrate 1 of Ga.sub.0.82 Al.sub.0.18 As and the optical waveguide layer 2 of GaAs, by forming a curved ridge pattern on the optical waveguide layer 2 which had a thickness of 0.8 .mu.m, and by etching the waveguide layer 2 to leave ridges of a depth of 0.5 .mu.m and a width of 3 .mu.m using an ion-milling apparatus. In other words, the ridges were constructed so as to have a width of 3 .mu.m, a height of 0.5 .mu.m above the waveguide layer, and a height of 0.8 .mu.m above the substrate. The ridges were curved with a radius of curvature from 0.7 mm to 0.5 mm. The transmission losses in this optical integrated circuit were measured using an He-Ne laser beam with an optical wavelength of 1.15 .mu.m. The results thereof are plotted in FIG. 6, the losses are the sums of the curvature losses and scattering losses. For smaller radii of curvature, curvature losses dominate the optical scattering losses so that the loss increases linearly, as indicated by a straight line in FIG. 6. At a radius of curvature of about 0.3 mm, however, the losses diverge from this linear change. This discrepancy is apparently caused by optical scattering. As shown in FIG. 3, there are more portions of curved optical waveguide when the pattern of the optical integrated circuit is complicated. A radius of curvature of about 1 mm is necessary to provide a high-density pattern, although the ridges do not need to have an extremely small radius of curvature such as 0.1 mm, as shown in FIG. 6. Therefore, a reduction in optical scattering is a major concern. In order to reduce the radius of curvature, trials have been made in which the irregularities of the ridge sides are coated with a substance such as that which functions as the cladding of optical fibers, to reduce the effect of the irregularities. This effect is reduced because the difference between the squares of the refractive indices of the ridges and the surrounding medium becomes small, and the scattering losses are proportional to that difference, as mentioned before. Another reason is that the effects of dust, etc., caught by the sides are removed. Since the refractive index of LiNbO.sub.3 is about 2.2, for example, a transmission loss of about 35% is ameliorated when the surrounding material is SiO.sub.2 glass compared with that of air. This effect is employed in practice. The characteristics required of the coating film are: (1) it coats the irregularities of the sides adequately, (2) its optical transmission losses are small, (3) it generates no strain in the ridges, and (4) it has a refractive index near that of the ridges, i.e., n.sub.1 .gtorsim.n.sub.3.
At present, the coating of MOS integrated circuits is prepared by sputtering SiO.sub.2 or the like (Rib waveguide switches with MOS electro-optic control for monolithic integrated optics in GaAs-Al.sub.x Ga.sub.1-x As., Appl. Opts. 17, No. 16, August 1978, pp 2548 to 2555). This method, however, fails to satisfy conditions (1) and (4) required for optical integrated circuits. Condition (4) is obvious. For condition (1), sputtering or similar methods are not sufficient for smoothing the irregularities on the sides. When the irregularities in the ridge sides vary so much in the heightwise direction that they form "caves", they cannot be completely coated, which means that an air layer (i.e., a layer with a refractive index of 1) remains and increases the optical scattering losses. Although some prior-art optical integrated circuits are mentioned in "Directional Coupler Switch in Molecular-beam Epitaxy GaAs.", Appl. Phys. Lett. 34 (11), 1, June 1979, pp 755 to 757, there is no prior-art example relating to the present invention.